5 ways to customize Spring MVC JSON/XML output

01 Apr 2018, 22:34

Spring MVC / REST

To adhere to guidelines or requirements, API designers may want to control how JSON/XML responses are formatted. Spring Web makes use of Jackson to perform JSON/XML serialization. Therefore, to customize our output format, we must configure the Jackson processor. Spring Web offers XML-based or Java-based approaches to handling configuration. In this article, we will look at the Java-based configuration.

Enable XML output

First of all, if you have created your project through https://start.spring.io/ you need add the following dependency to your project because there is no XML serialization by default:

<dependency>
	<groupId>com.fasterxml.jackson.dataformat</groupId>
	<artifactId>jackson-dataformat-xml</artifactId>
</dependency>

ok, now we can investigate the output customization process. Imagine we have this REST controller:

@RestController
@RequestMapping("/users")
public class UsersResource {

    @RequestMapping("/{userID}")
    public User getByID(@PathVariable("userID") int userID){
        // I ignore fetching user from database
        // and just return a hardcoded user
        User user = new User("mostafa",null,18);
        return user;
    }

}

And User is defined like this:

public class User {

    private String firstName;
    private String lastName;
    private int age;

    public User(String firstName, String lastName, int age) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.age = age;
    }

    public String getFirstName() {
        return firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public int getAge() {
        return age;
    }
}

If you start your service and call your service like this:

curl http://localhost:8080/users/15

You will get the output something like this:

{"firstName":"mostafa","lastName":null,"age":18}

Now let’s back to our problem. How we can change the generated output? For example change firstName to first_name and remove fields with null values like lastName.

Solution 1> Use properties

You can configure the ObjectMapper and XmlMapper instances by using the environment. Jackson provides an extensive suite of simple on/off features that can be used to configure various aspects of its processing. For example, to enable pretty print, add this line to the application.properties file:

spring.jackson.serialization.indent_output=true

Here is the valid properties:

Solution 2> Use Jackson2ObjectMapperBuilderCustomizer

You can customize the default Jackson2ObjectMapperBuilder using Jackson2ObjectMapperBuilderCustomizer. Just create a new class annotated with @Configuration and return an instance of Jackson2ObjectMapperBuilderCustomizer with @Bean annotation:

@Configuration
public class OutputConfiguration {
    @Bean
    public Jackson2ObjectMapperBuilderCustomizer customJson(){
        return builder -> {
            // human readable
            builder.indentOutput(true);
            // exclude null values
            builder.serializationInclusion(JsonInclude.Include.NON_NULL);
            // all lowercase with under score between words
            builder.propertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);
        };
    }
}

Solution 3> Use ObjectMapper

This method replace default ObjectMapper instance:

@Configuration
public class OutputConfiguration {
    @Bean
    @Primary
    public ObjectMapper customJson(){
        return new Jackson2ObjectMapperBuilder()
                   .indentOutput(true)
                   .serializationInclusion(JsonInclude.Include.NON_NULL)
                   .propertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE)
                   .build();
    }
}

Solution 4> Use Jackson2ObjectMapperBuilder

Like the above code, but just return the builder:

@Configuration
public class OutputConfiguration {
   @Bean
    public Jackson2ObjectMapperBuilder customJson() {
        return new Jackson2ObjectMapperBuilder()
                .indentOutput(true)
                .serializationInclusion(JsonInclude.Include.NON_NULL)
                .propertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE);
    }
}

Solution 5> Use MappingJackson2HttpMessageConverter

@Bean
    public MappingJackson2HttpMessageConverter customJson(){
        return new MappingJackson2HttpMessageConverter(
                new Jackson2ObjectMapperBuilder()
                        .indentOutput(true)
                        .serializationInclusion(JsonInclude.Include.NON_NULL)
                        .propertyNamingStrategy(PropertyNamingStrategy.SNAKE_CASE)
                        .build()
        );
}

That’s it. Do you know the other ways to customize JSON/XML output? If you know let me know.
source: Spring Mvc documentation